hashes on restricted domains: random functions or permutations?

[James A. Donald]

Travis H. wrote:
> So I was reading about the OTP system (based on S/Key) described in RFC 
> 2289.
> It basically hashes a secret several times (with salt to individualize
> it) and stores
> the value that the correct password will hash to.
> 
> Now my question is, if we restrict ourselves to, say, 160-bit inputs, is 
> SHA-1
> a permutation, or do collisions exist?  If there are collisions, then 
> iterating
> the hash could lead to fewer possible values each time, potentially 
> converging
> on a set of inputs that form a permutation and are closed under 
> composition.
> 
> Is that correct?

Yes.

> What are the expected sizes of such sets?

More relevant is how many iterations it takes to get to a significantly 
smaller set.

> Is it worth worrying about?

No.



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